178 The Living Plant 



around themselves, thus forcing themselves apart against the 

 resistance of their own cohesion. This explains how it is that 

 membranes, and all bodies of similar constitution, like wood, can 

 forcibly absorb water throughout all of their structure, and 

 swell up in the process, the requisite energy being supplied by 

 the adhesive attraction between water and wood. This inter- 

 micellar absorption of water is called imbibition, and is represented 

 in the accompanying diagram (figure 59). But why, by the way, 

 are the micellae not driven entirely apart by the water, thus 

 making the membrane completely soluble therein? The reason 

 is believed to be this, that while the adhesive attraction of 

 micellae for water, and the cohesive attraction of the micellae for 

 one another, are, like the attraction of a magnet for its armature, 

 strongest when the parts are the closest and weaker with increas- 

 ing distance apart, the adhesion is supposed to weaken with 

 distance more rapidly than the cohesion; hence, although the 

 adhesion between micellae and water is at first stronger than the 

 cohesion of the micellae (thus drawing in some films of the water) 

 there soon comes a point at which the rapidly-lessening adhesion 

 between water and micellae exactly balances the slowly-lessening 

 cohesion between 'the micellae, and this point of equilibrium is 

 that where the membrane is saturated with water and swollen 

 its greatest, as supposed to be represented in figure 59, 6. In this 

 condition the intermicellar spaces will possess a certain definite 

 size, differing, of course, with the nature of the membrane; and 

 in these different sizes we find the simplest explanation of the 

 different behavior of the types of membranes, for the semi- 

 permeable would be one with intermicellar spaces too small to 

 allow the sugar or other large molecules to pass, while giving free 

 transit to the much smaller water molecules, while the permeable 

 has large enough spaces to permit both kinds to pass. The case 

 in reality, however, is not quite so simple as this, for plenty of 

 facts show that adhesion or solution relations between dissolved 

 substance and the membrane play also a part. Moreover, the 



